Streamlining Exoplanet Validation

byPaul GilsteronFebruary 22, 2018

Between Kepler and the ensuing K2 mission, we’ve had quite a haul of exoplanets. Kepler data have been used to confirm 2341 exoplanets, with NASA declaring 30 of these as being less than twice Earth-size and in the habitable zone. K2 has landed 307 confirmed worlds of its own. K2 offers a different viewing strategy than Kepler’s fixed view of over 150,000 stars. While the transit method is still at work, K2 pursues a series of observing campaigns, its fields of view distributed around the ecliptic plane, and with photometric precision approaching the original.

Why the relationship with the ecliptic? Remember that what turned Kepler into K2 was the failure of two reaction wheels, the second failing less than a year after the first. Working in the ecliptic plane minimizes the torque produced by solar wind pressure, thus minimizing pointing drift and allowing the spacecraft to be controlled by its thrusters and remaining two reaction wheels. Each K2 campaign is limited to about 80 days because of sun angle constraints.

Image: After detecting the first exoplanets in the 1990s, astronomers have learned that planets around other stars are the rule rather than the exception. There are likely hundreds of billions of exoplanets in the Milky Way alone. Credit: ESA/Hubble/ESO/M. Kornmesser.

More K2 planets have now turned up in an international study led by Andrew Mayo (National Space Institute, Technical University of Denmark). The research, underway since the first release of K2 data in 2014, uncovered 275 planet candidates, of which 149 were validated. 56 of the latter had not previously been detected, while 39 had already been identified as candidates, and 53 had already been validated, with one previously classed as a false positive.

Overall, the work increases the validated K2 planet sample by almost 50 percent, while increasing the K2 candidate sample by 20 percent. What stands out here is not so much the trove of new planets but the validation techniques brought to bear, which were applied to a large sample as part of a framework developed to increase validation speed. From the paper:

This research will also be useful even after the end of the K2 mission. The upcoming TESS mission (Ricker et al. 2015) is expected to yield more than 1500 total exoplanet discoveries, but it is also estimated that TESS will detect over 1000 false positive signals (Sullivan et al. 2015). Even so, one (out of three) of the level one baseline science requirements for TESS is to measure the masses of 50 planets with Rp < 4 R⊕. Therefore, there will need to be an extensive follow-up program to the primary photometric observations conducted by the spacecraft, including careful statistical validation to aid in the selection of follow-up targets. The work presented here will be extremely useful in that follow-up program, since only modest adjustments will allow for the validation of planet candidate systems identified by TESS rather than K2.

The work involves not only analysis of the K2 light curves but also follow-up spectroscopy and high contrast imaging involving ground-based observation of candidate host stars. The processes of data reduction, candidate identification, and statistical validation described here using a statistical validation tool called vespa clearly have application well beyond K2.

“Photometric survey data from the Kepler mission have been used to discover and characterize thousands of transiting exoplanet and eclipsing binary (EB) systems. These discoveries have enabled empirical studies of occurrence rates which reveal that exoplanets are ubiquitous and found in a wide variety of system architectures and physical compositions. Because the detection strategy of these missions is most sensitive to short orbital periods, the vast majority of these objects reside within 1 AU of their host star. Although other detection techniques have successfully identified exoplanets at wider orbits beyond the snow lines of their respective host stars (e.g., radial velocity, microlensing, direct imaging), occurrence rates within this population remain poorly constrained. As such, identifying long period objects (LPOs) from archival Kepler and K2 data is valuable from both a statistical and theoretical standpoint, particularly for massive gas giants which are thought to heavily influence the formation and evolution dynamics of their respective systems. Here we present a catalog of 164 single transit and eclipse candidates detected during a comprehensive survey of all currently available K2 data.”

Are all detections by Kepler verifiable (at least in principle) by other current instruments? Or is Kepler uniquely sensitive in ways that may make some confirmations (or rejections) impossible for now?

making the case that when faced with huge numbers of candidates, this may be the only way to proceed given the strain on our resources to do radial velocity studies on each. That eliminates astronomical follow-ups but gives no information about mass. Just what the limits of RV are with smaller Kepler worlds is a question I can’t answer, but I’m hoping one of our resident astronomers will.

(Submitted on 21 Feb 2018)
This paper puts forward a possible new indicator for the presence of moderately advanced civilizations on transiting exoplanets. The idea is to examine the region of space around a planet where potential geostationary or geosynchronous satellites would orbit (herafter, the Clarke exobelt). Civilizations with a high density of devices and/or space junk in that region, but otherwise similar to ours in terms of space technology (our working definition of “moderately advanced”), may leave a noticeable imprint on the light curve of the parent star. The main contribution to such signature comes from the exobelt edge, where its opacity is maximum due to geometrical projection. Numerical simulations have been conducted for a variety of possible scenarios. In some cases, a Clarke exobelt with a fractional face-on opacity of ~1E-4 would be easily observable with existing instrumentation. Simulations of Clarke exobelts and natural rings are used to quantify how they can be distinguished by their light curve.

How about an artificial solid ring? It would have a different optical characteristics from natural rings like Saturn and also the Clarke Exobelt. Depending on how it was made it would most likely have reflective spikes on each side of the planet.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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